1. Field of the Invention
The present invention relates generally to a control method and device for echo canceller and more particularly, to an echo canceller control method and an echo canceller control device for use in hands-free telephones, live videoconference systems and the like.
2. Prior Art
In echo cancellers applied to hands-free telephones and employing adaptive filters, an adaptation operation of the adaptive filter is normally performed under a single talk condition. However, when a double talk condition (two-way simultaneous conversations) occurs, the adaptation operation is performed in a manner such as to remove not only an echo signal but also a talker""s speech, resulting in confused estimate of an echo path impulse response. Hence, a learning operation must be suspended in the presence of the double talk condition.
In past, a variety of techniques have been proposed for detecting the double talk condition by using the input/output signal level, correlation function and the like. FIG. 13 shows a prior-art echo canceller 2 including a double talk detecting circuit 1 (see FIG. 2 of Japanese Unexamined Patent Publication No. 5(1993)-102887).
In the double talk detecting circuit 1, a level of a digital receive path signal RS is detected by an absolute value detecting circuit 3 and a differential circuit 5 while a level of a digital transmit path signal TS is detected by an absolute value detecting circuit 4 and a differential circuit 6.
Next, a comparator 7 performs an arithmetic operation of R=log{(a level of receive path signal RS)/(a level of transmit path signal TS)} for determination of a ratio R of the detected signal levels, which ratio is compared with a predetermined threshold value.
When the level ratio R becomes smaller than the threshold value, the double talk condition is determined to be present. In a period during which the double talk condition is determined to be present, the adaptation operation of an adaptive filter (ADF) 8 is suspended. This prevents the echo canceller 2 from making a false estimate of an impulse response of an echo path EP under the double talk condition, thus ensuring reliable echo cancellation operations.
In such an echo canceller 2, however, when the transmit path signal level becomes lower in conjunction with the movement of a near end talker, the level ratio R of the transmit path signal versus the receive path signal exceeds the threshold value. This may lead to a false determination of the single talk condition despite the actual presence of the double talk condition.
Thus, the prior-art double talk detecting circuit 1 is adversely affected by variations of the signal level due to the movement of the near end talker, a line failure and the like, suffering degraded accuracies of the double talk detection. Hence, unreliable echo cancellation operations result.
In view of the foregoing, it is an object of the invention to provide a control method and device for controlling the echo canceller which ensure the reliable echo cancellation operations.
In accordance with the invention, a first method of controlling the echo canceller for providing an echo cancellation signal by subtracting an echo replica signal either from a transmit signal or a receive signal, which method comprises the steps of: a first step of performing a speech detection based on the echo cancellation signal provided by the echo canceller; a second step of detecting a double talk condition based on a result of the speech detection performed by the first step; and a third step of controlling a learning operation of the echo canceller based on a result of the double talk detection performed by the second step.
In the first step, the speech detection is achieved by, for example, comparing a level of the echo cancellation signal provided by the echo canceller with a threshold value adaptively determined according to a level of background noise.
In the second step, the double talk condition is determined to be present when, for example, the echo cancellation signal provided by the echo canceller is determined to include speech and continues to indicate the presence of the speech for more than a predetermined period of time.
In the third step, the learning operation of the echo canceller is suspended while the double talk condition is detected by the second step.
In accordance with the invention, a second method of controlling an echo canceller for providing an echo cancellation signal by subtracting an echo replica signal either from a transmit signal or a receive signal, which method comprises the steps of: a first step of determining whether a learning operation of the echo canceller is permitted or not based on either of the transmit signal and the receive signal that is not removed of echo by the echo canceller; a second step of performing a speech detection based on the echo cancellation signal provided by the echo canceller; a third step of detecting a double talk condition based on a result of the speech detection performed by the second step; and a fourth step of controlling the learning operation of the echo canceller based on a determination made by the first step and a result of the double talk detection performed by the third step.
In the second step, the speech detection is achieved by, for example, comparing a level of the echo cancellation signal provided by the echo canceller with a threshold value adaptively determined according to a level of background noise.
In the third step, the double talk condition is determined to be present when, for example, the echo cancellation signal provided by the echo canceller is determined to include speech and continues to indicate the presence of the speech for more than a predetermined period of time.
In the fourth step, the learning operation of the echo canceller is suspended while the first step determines the learning operation of the echo canceller to be inhibited or while the double talk condition is detected by the third step.
In accordance with the invention, a third method of controlling an echo canceller for providing an echo cancellation signal by subtracting an echo replica signal either from a transmit signal or a receive signal, which method comprises the steps of: a first step of performing a speech detection based on the echo cancellation signal provided by the echo canceller; a second step of performing a speech detection based on either of the transmit signal and the receive signal that is not removed of echo by the echo canceller; a third step of performing a double talk detection based on a result of the speech detection performed by the first step and a result of the speech detection performed by the second step; and a fourth step of controlling a learning operation of the echo canceller based on a result of the double talk detection performed by the third step.
In the first step, the speech detection is achieved by, for example, comparing a level of the echo cancellation signal provided by the echo canceller with a first threshold value adaptively determined according to a level of background noise.
In the second step, the speech detection is achieved by, for example, comparing a level of either of the transmit signal and the receive signal that is not removed of the echo by the echo canceller with a second threshold value adaptively determined according to a level of background noise.
In the third step, the double talk condition is determined to be present when, for example, both the echo cancellation signal provided by the echo canceller and either of the transmit signal and the receive signal that is not removed of the echo by the echo canceller are determined to include speeches and continue to indicate the presence of the speeches for more than a predetermined period of time.
In the fourth step, the learning operation of the echo canceller is suspended while the double talk condition is detected by the third step.
In accordance with the invention, a fourth method of controlling an echo canceller for providing an echo cancellation signal by subtracting an echo replica signal either from a transmit signal or a receive signal, which method comprises the steps of: a first step of determining whether a learning operation of the echo canceller is permitted or not based on either of the transmit signal and the receive signal that is not removed of echo by the echo canceller; a second step of performing a speech detection based on the echo cancellation signal provided by the echo canceller; a third step of performing a speech detection based on either of the transmit signal and the receive signal that is not removed of the echo by the echo canceller; a fourth step of detecting a double talk condition based on a result of the speech detection performed by the second step and a result of the speech detection performed by the third step; and a fifth step of controlling the learning operation of the echo canceller based on a determination made by the first step and a result of the double talk detection performed by the fourth step.
In the second step, the speech detection is achieved by, for example, comparing a level of the echo cancellation signal provided by the echo canceller with a first threshold value adaptively determined according to a level of background noise.
In the third step, the speech detection is achieved by, for example, comparing a level of either of the transmit signal and the receive signal that is not removed of the echo by the echo canceller with a second threshold value adaptively determined according to a level of background noise.
In the fourth step, the double talk condition is determined to be present when, for example, both the echo cancellation signal provided by the echo canceller and either of the transmit signal and the receive signal that is not removed of the echo by the echo canceller are determined to include speeches and continue to indicate the presence of the speeches for more than a predetermined period of time.
In the fifth step, the learning operation of the echo canceller is suspended while the first step determines the learning operation of the echo canceller to be inhibited or while the double talk condition is detected by the fourth step.
In accordance with the invention, a fifth method of respectively controlling an acoustic echo canceller for providing a first echo cancellation signal by subtracting a first echo replica signal from a transmit signal and a hybrid echo canceller for providing a second echo cancellation signal by subtracting a second echo replica signal from a receive signal, which method comprises the steps of: a first step of determining whether a learning operation of the acoustic echo canceller is permitted or not based on a level of the second echo cancellation signal provided by the hybrid echo canceller; a second step of determining whether a learning operation of the hybrid echo canceller is permitted or not based on a level of the first echo cancellation signal provided by the acoustic echo canceller; a third step of performing a speech detection based on the level of the first echo cancellation signal; a fourth step of performing a speech detection based on the level of the second echo cancellation signal; a fifth step of detecting a double talk condition based on a result of the speech detection performed by the third step and a result of the speech detection performed by the fourth step; a sixth step of controlling the learning operation of the acoustic echo canceller based on a determination made by the first step and a result of the double talk detection performed by the fifth step; and a seventh step of controlling the learning operation of the hybrid echo canceller based on a determination made by the second step and the result of the double talk detection performed by the fifth step.
For example, the fifth step comprises a step of determining that a single talk condition is present if at least one of the results of the speech detections performed by the third step and the fourth step indicate the absence of speech; a step of calculating a duration of speeches if both the results of the speech detections performed by the third step and the fourth step indicate the presence of the speeches; a step of determining that an indeterminate condition is present if the duration of the speeches is less than a predetermined period of time; and a step of determining that the double talk condition is present if the duration of the speeches is not less than the predetermined period of time.
It is preferred that the fifth step further comprises a step of backing up an adaptive filter coefficient provided by the. learning operation of the acoustic echo canceller and an adaptive filter coefficient provided by the learning operation of the hybrid echo canceller, respectively, when the single talk condition is present; and a step of performing echo cancellation operations by using the adaptive filter coefficients backed up by the acoustic echo canceller and the hybrid echo canceller, respectively, when the double talk condition or the indeterminate condition is present.
In accordance with the invention, a first control device for controlling an echo canceller for providing an echo cancellation signal by subtracting an echo replica signal either from a transmit signal or a receive signal, which control device comprises: judgment means for determining whether a learning operation of the echo canceller is permitted or not based on either of the transmit signal and the receive signal that is not removed of echo by the echo canceller; first speech detecting means for performing a speech detection based on the echo cancellation signal provided by the echo canceller; second speech detecting means for performing a speech detection based on either of the transmit signal and the receive signal that is not removed of the echo by the echo canceller; double talk detecting means for detecting a double talk condition based on a result of the speech detection performed by the first speech detecting means and a result of the speech detection performed by the second speech detecting means; and control means for controlling the learning operation of the echo canceller based on a determination made by the judgment means and a result of the double talk detection performed by the double talk detecting means.
In accordance with the invention, a second echo canceller control device for respectively controlling an acoustic echo canceller for providing a first echo cancellation signal by subtracting a first echo replica signal from a transmit signal and a hybrid echo canceller for providing a second echo cancellation signal by subtracting a second echo replica signal from a receive signal, which control device comprises: first judgment means for determining whether a learning operation of the acoustic echo canceller is permitted or not based on a level of the second echo cancellation signal provided by the hybrid echo canceller; second judgment means for determining whether a learning operation of the hybrid echo canceller is permitted or not based on a level of the first echo cancellation signal provided by the acoustic echo canceller; first speech detecting means for performing a speech detection based on the level of the first echo cancellation signal; second speech detecting means for performing a speech detection based on the level of the second echo cancellation signal; double talk detecting means for detecting a double talk condition based on a result of the speech detection performed by the first speech detecting means and a result of the speech detection performed by the second speech detecting means; first control means for controlling the learning operation of the acoustic echo canceller based on a determination made by the first judgment means and a result of the double talk detection performed by the double talk detecting means; and second control means for controlling the learning operation of the hybrid echo canceller based on a determination made by the second judgment means and the result of the double talk detection performed by the double talk detecting means.
According to the first echo canceller control method as defined by the invention, the speech detection is performed based on the echo cancellation signal provided by the echo canceller so that possibilities of false detection mistaking the echo for speech can be decreased. Inasmuch as the double talk condition is detected based on such a reliable result of the speech detection, the double talk condition is not falsely detected as the single talk condition. Therefore, the echo canceller is prevented from making a false estimate of the echo path.
In addition, the speech detection is achieved by comparing the level of the echo cancellation signal provided by the echo canceller with the threshold value adaptively determined according to the level of the background noise whereby the speech detection accuracies are prevented from being degraded by environmental changes.
Further, the presence of the double talk condition is determined when the echo cancellation signal provided by the echo canceller is determined to include speech and continue to indicate the presence of the speech for more than the predetermined period of time. This makes it possible to discriminate the double talk condition from an event caused by a degraded echo canceling performance due to the echo path changes. Hence, the accuracies of the double talk detection can be improved.
According to the second and fourth echo canceller control methods or to the first echo canceller control device as defined by the invention, whether the learning operation of the acoustic echo canceller interposed in the transmit path is permitted or not is determined based on the receive signal. On the other hand, whether the learning operation of the hybrid echo canceller interposed in the receive path is permitted or not is determined based on the transmit signal. When the level of the inputted signal is too low for the echo canceller to achieve learning, the learning operation of the echo canceller can be suspended irrespective of the presence of the double talk condition. This is effective to avoid the learning operation with a low degree of reliability.
According to the third and the fourth echo canceller control methods and the first echo canceller control device as defined by the invention, the acoustic echo canceller interposed in the transmit path also serves to detect speech in the receive signal. On the other hand, the hybrid echo canceller interposed in the receive path also serves to detect speech in the transmit signal. The result of such speech detection may also be utilized for enhancing the accuracy of the double talk detection, thus contributing to the improved reliability of the learning operation of the echo canceller.
According to the fifth echo canceller control method or the second echo canceller control device as defined by the invention, whether the learning operation of the acoustic echo canceller is permitted or not is determined based on the second echo cancellation signal provided by the hybrid echo canceller. On the other hand, whether the learning operation of the hybrid echo canceller is permitted or not is determined based on the first echo cancellation signal provided by the acoustic echo canceller.
This enables it to suspend the learning operations of the acoustic echo canceller and the hybrid echo canceller irrespective of the presence of the double talk condition when the levels of the transmit signal and the receive signal, which have been removed of echo, are too low for the acoustic and hybrid echo cancellers to achieve the learning. Hence, the learning operations with low degrees of reliability can be prevented.
Inasmuch as the speech detections are performed based on the levels of the first and second echo cancellation signals, respectively, the possibilities of false detection mistaking the echo for speech can be decreased. Thus, the double talk condition is detected based on the speech detections with high degrees of reliability so that the double talk condition is not falsely detected as the single talk condition. Hence, an increased accuracy of the double talk detection is accomplished. Consequently, the echo canceller is prevented from making the false estimate of the echo path, accomplishing an enhanced reliability of the learning operation.